The systems and methods according to this disclosure are directed to reducing data recovery errors including low frequency distortions that can degrade the quality of timing loop information and/or detection of address information in digital data recording systems, particularly those in which data is recorded on recordable or re-recordable optical disc data storage media.
With a need to provide removable non-volatile data storage media on which increasing amounts of data can be recorded and/or re-recorded, optical disc data storage media have proven both comprehensive and flexible enough to support expanding data storage requirements. Optical disc data storage relates to placing data on a recordable, re-recordable and/or readable surface of an optical disc. In general, to record data on, or recover data previously recorded on, an optical disc, a light beam is used to scan the surface of the optical disc using systems specifically designed for such data recovery. Currently-available recordable or re-recordable optical disc data storage media include: CD-R (Compact Disc-Recordable), DVD-R (Digital Video Disc-Recordable), DVD-RW (DVD-Rewritable), DVD+R (Writable Optical Disc), DVD+RW (Rewritable Optical Disc), DVD-RAM (DVD-Random Access Memory), and new technology higher density recordable or re-recordable optical data storage discs known as BD technology, such as HD-DVD (High Density-DVD or High Definition-DVD) and Blu-ray Discs.
Differing methodologies are, therefore, required by which, when data is recorded or re-recorded to such optical disc data storage media, a timing synchronization signal is provided, monitored and adjusted in order that the readback, or data retrieval, system is cued to retrieve the discretely recorded or re-recorded data from a discrete portion of the disc at the precise speed with which the data was recorded.
A conventional optical disc formatted for land-groove recording is shown in exemplary embodiment in FIG. 1. Digital data is stored on such optical discs in the form of arrangements of data marks in spiral tracks. As shown in FIG. 1, grooves 100 and lands 110 are formed by means of a guide channel cut into the surface of a disc substrate. A recording layer (not identified) is then formed over the entire disc surface including the surfaces of the grooves 100 and the lands 110. The grooves 100 and the lands 110 each form continuous recording tracks on the disc. Data recording and reproducing are accomplished with such an optical disc storage medium by scanning the groove recording track or the land recording track with a focused light beam spot of an optical disc drive device, as shown in, and described in connection with, FIGS. 2 and 3 below. It should be noted that, in some formats, data is recorded both on lands and grooves. In other formats, data is recorded only in the grooves.
FIG. 2 illustrates an exemplary conventional apparatus for implementing a process to write data to an optical disc data storage medium. As shown in FIG. 2, an input stream of digital information 200 is converted using an encoding/modulating unit (encoder/modulator) 205 into a drive signal 210 for a light source such as a laser source 220. The laser source 220 emits a light beam 225 that is directed toward, and focused onto, a recording surface 250 of an optical disc data storage medium 245. The focusing of the light beam 225 typically involves an illumination optics unit 230 to produce a very precise scanning spot 240. The diameter of the scanning spot 240 precisely coincides with the width of the groove and/or the land in the optical disc data storage medium 245. In order to accommodate more information on a single optical disc data storage medium, the lands and the grooves are made individually thinner in a radial direction requiring that the illumination optics unit 230 ever-more-precisely focus the scanning spot 240, thereby reducing the diameter of the scanning spot 240. As the surface 250 of optical disc data storage medium 245 is rotated under the scanning spot 240, energy from the scanning spot 240 is absorbed by a surface treatment on the surface 250 of the optical disc data storage medium 245 through heating of a small, localized region of the surface 250. The reflective properties of the surface 250 of the optical disc data storage medium 245 are thus locally discretely altered in accordance with, and to reflect recording of, the input data stream 200. Modulation of the light beam 225 is synchronous with the drive signal 210, so a circular track of data marks is formed as newly written data 235 as the surface 250 rotates.
FIG. 3 illustrates an exemplary conventional apparatus for implementing a process to read data from an optical disc data storage medium. As shown in FIG. 3, a light beam 305 from a light source such as a laser source 300 (which may be the same as the writing laser source 220 shown in FIG. 2) is directed through a beam splitter device 310 into an illumination optics unit 320 (which may be the same as illumination optics unit 230 shown in FIG. 2) to focus the light beam 305 onto a surface 340 of the recorded optical disc data storage medium 335. As previously-recorded data marks to be read 345 pass under a scanning spot 350, light is reflected toward the illumination optics unit 320. Reflected light is collected by the illumination optics unit 320 and directed by the beam splitter 310 toward a collector of a data optics unit 360. The data optics unit 360 converges the reflected light onto one or more detectors in a light detector array 370. Detectors in the detector array 370 convert the reflected light into a current modulated signal 375. This collected current modulated signal 375 is amplified and/or decoded in an amplifier/decoder unit 380 to produce an output data stream 385 that corresponds to the previously-recorded data marks to be read 345 from the surface 340 of the optical disc data storage medium 335.
In data storage applications, inclusion of synchronizing marks, also referred to as timing information marks, and physical location information, are essential for recording data at a certain location on the optical disc data storage medium to facilitate, among other capabilities, finding the data location at a later time. A sector number, sector type and a land track/groove track can be recognized from the address information. In other words, the address information provides information for finding a specified sector to record/reproduce data to/from a certain location in an optical disc data storage medium.
When data is stored randomly on an optical disc data storage medium, various methods are included in the recording process to encode address and timing synchronization information. One method includes recording such information on, for example, a non-data area or non-recording area of the optical disc data storage medium by forming embossed pits separately from data recording sectors. These pits are pre-formed and then during the recording process recorded with non-data information to facilitate data identification and location, and timing synchronization for readback. A drawback to this method, however, is that these pre-pitted areas reduce the effective recording area of the optical disc data storage medium. Another method employed, particularly for higher density recording applications, is referred to as “wobbling” in which the lands and grooves of an optical disc data storage medium are pre-wobbled, in a radial direction, at a specific frequency.
FIG. 4 illustrates an exemplary embodiment of a conventional optical disc data storage medium 400 into which a predetermined reference wobble 410 is physically encoded, i.e., the grooves (and/or lands) of the recordable surface of the optical disc data storage medium are physically pre-wobbled at a given frequency. As shown in FIG. 4, a sinusoidal wave with an amplitude in a radial direction is physically introduced into the grooves. When data is recorded, frequency or phase modulation is then performed around this reference wobble. Alternatively, land pre-pits may be performed on the wobble signal to carry data, such as address information. The address information is encoded in a modulated wobble signal, and introduced, for example, with a measurable, modulated signal offset when the data is recorded to the optical disc data storage medium 400. During readback of, and/or other recovery of data from, the optical disc data storage medium, an address of the pre-recorded data may be identified by demodulating the recorded wobble signal from the reference wobble. Timing synchronization information for, for example, controlling rotation speed of the optical disc data storage medium precisely to facilitate clear readback or data recovery may be implemented by demodulating a modulated wobble signal in comparison to a wobble reference signal. Precise timing synchronization for readback and/or other data recovery is implemented through use of a timing loop such as, for example, a phase-locked loop to control a readback speed of data from a sector of the optical disc data storage medium in response to the demodulated recorded wobble signal.
Errors in detecting and properly employing a wobble signal may be introduced by, for example, presence of low frequency distortion and/or a DC-offset component in the wobble signal.